Transmission networks serve for the transport of user signals, commonly referred to as tributary signals, in the form of multiplexed transmission signals. A transmission network consists of a number a physically interconnected network elements such as add/drop multiplexers, terminal multiplexers, cross-connects and line equipment. The physical interconnection between two network elements is referred to as a section or link while the route a particular tributary takes through the transmission network from end to end is known as a path. Although in the context of path protection, the term path is commonly also used for a segment of a path, the above specification uses the more appropriate terminology and distinguishes between entire paths (from end to end) and path segments without path termination function. A path is represented by a multiplexing unit like a virtual container (VC-N) with its associated path overhead (POH) in SDH (Synchronous Digital Hierarchy). Conversely, a section is represented by an entire transmission frame like a synchronous transport module (STM-N) with its associated section overhead (SOH).
A very basic aspect of transmission networks is availability of service. Hence, a transmission network itself or the combination of network and network management needs to provide the means and facilities to ensure sufficient availability. Typically, these network mechanisms are distinguished in protection and restoration. The principle of both is to redirect traffic of a failed link to a spare link. Restoration means network management interaction to determine an alternative route through the network while protection uses dedicated protection resources already available and established in the network for this purpose.
Protection mechanisms are widely used and standardised. For example ITU-T G.841 and G.783 describe several protection mechanisms for SDH networks and G.709, G.798 describe corresponding protection mechanisms for OTNs (Optical Transport Networks).
Section protection refers to the protection of a physical link between two network elements. Known section protection mechanisms include 1+1 MSP (Multiplex Section Protection), 1:1 MSP, 1:n MSP and MS-SPRING (Multiplex Section Shared Protection Ring). 1+1 MSP means that two redundant links are provided between two network elements and that all traffic is permanently bridged to the protection links so that the receiving network element can choose the better of the two received signals. 1:1 MSP means that the protection link can be used for extra traffic that is discarded instantly in the case of a failure of the working link and the protected traffic bridged from the failed working to the protection link. 1:n MSP denotes a protection mechanism where one protection link serves to protect n working links. In the case of a failure, traffic from the failed link is bridged to the protection link. 1:1 MSP, 1:n MSP and MS-SPRING require a protocol to communicate a failure from sink to source and synchronize switch-over. SDH uses K1/K2 bytes in the section overhead (SOH) for this purpose.
Conversely, path protection refers to the protection of a path or a segment thereof. G.783 describes a 1+1 path protection mechanism for SDH, which is known as SNCP (Sub-Network Connection Protection). Like for 1+1 MSP, the protected traffic is permanently bridged to a dedicated protection path. A trail termination function required for path level protection is discussed in ITU-T study group 15 draft G.gps (CD-GPS01). An automatic protection protocol on path level is still under discussion and not yet defined, so that 1:1 or 1:n protection on path level is not possible today.
Thus, existing path protection mechanisms require a 100% spare capacity of resources for protection in the network but allow a very fast masking of the failure in terms of availability, typically in less than 50 ms.
Restoration mechanisms are introduced in network management in order to use the spare resources of a network for traffic protection in a flexible way and therefore to reduce the necessary amount of spare resources in a meshed network.
Restoration mechanisms are more stringent in the usage of spare capacity but however, provide a masking of the failure at a lower speed, typically in the range of a few seconds, as completely new paths through the network need to be established by the network management system after the occurrence of a failure. Therefore restoration is regarded as too slow for many applications.
It is therefore an object of the present invention to provide a more efficient and more flexible protection method on path level which allows masking of a failure within shorter time than known restoration methods while still requiring less spare resources in the network than traditional 1+1 path protection mechanisms do.